LG 50PC3D-UD Power Supply Repair

[tds-1541]

A friend give me an LG 50PC3D-UD (PDP50X30010) plasma TV for repair. The TV would turn on and the green front panel lamp would blink three times, then the power supply would go into some sort of protect mode and shut down. This happened irrespective of what connectors I had plugged into the supply. I started measuring voltages and all low voltages checked out (5v-Standby, 3.3v, both 5v rails, 6v, 12v, and 19v). In the high voltage section, the HorizontalGrid Voltage (VS) was reading 0v (should be 196v according to the service manual) and the Vertical Grid Voltage (VA) was reading 60v.

Based partially off of a suggestion in this thread I checked all MOSFETS and located Q855, a shorted SD20N60 power MOSFET. Upon removing this FET, it showed continuity to the source (S), gate (G), drain (D), while Q854 (same part) passed the "finger" test (touch a finger between source and gate after the gate has been charged). I ordered a replacement from Coppell and installed it. TV In addition to this, I replaced all bloated capacitors with Panasonic FM equivalents. Nine in total.

Now, with the power supply on my bench and my trusty fluke at my side, when I apply the 5v standby to the RL-ON (relay on) the power supply relays engage. I am then able to apply 5v to the M5V-ON and 3.3-ON pins and can verify that all low voltages are in spec. When I engage the high voltage section by applying 5v to the VS-ON pin, the power supply goes into protect mode only a moment later. I am able to read exactly 60v on VA, but now VS jumps past the 196V mark until the powers supply kicks into protect mode around 217v. I have tried adjusting the VS adjustment potentiometer to no avail.

Does anyone have any ideas? the manual entitled " lg_50pc1dr_troubleshooting_manual.pdf" has been of great help, but it does not have a full schematic for the power supply section. I read that a failed MOSFET can sometimes burn up the drive section, but this section seems to be protected with a zener diode (admittedly, I don't fully understand all principles involved as electronics are just a hobby of mine). There are two identical drive circuits in this section of the power supply (one for Q854 and one for Q855) and all components in them test out the same with a diode test, but I did not remove any of their components to test them. A Motorola MC33067P controls this circuit. Everything looks good upon visual inspection. Photos can be uploaded if need be.

Any thoughts would be appreciated!

[tds-1541]

Re: LG 50PC3D-UD Power Supply Repair

I was able to take some high resolution photos of the board. The board is just slightly too large to fit on my old flatbed scanner. The power supply part number is: YPSU-J012B REV 1.0 2300KEG003A-F.

The Motorola MC33067P switch mode controller appears to be controlled by two optocouplers. Following the traces, one appears to "enable" the unit by tying into pin 9 (Enable/UVLO Adjus) while the other appears to tie into pin 10 (fault). The circuit trips into protect so fast (around 1 second), but I'll try and measure the input and output of those optocouplers when the high voltage section is enabled.

I just wish I could tell if the issue was on the primary side or the secondary side. This isn't a situation where the unit needs a load to generate the proper voltages is it? There appear to be bleed resistors on the power supply and so I had amused that those would provide a minor load. Additionally, basic troubleshooting steps from other posts seem to indicate that the unit should continue to function with the VS and VA cable to the Y sustain disconnected.

Any thoughts here? Thanks in advance.

[dcapper]

Re: LG 50PC3D-UD Power Supply Repair

Are you testing the the power supply with the ZSUS and YSUS boards connected? If so try it without. You could have a problem with one of those boards causing the draw.

[tom66]

Re: LG 50PC3D-UD Power Supply Repair

I have never heard of Vs called the "horizontal grid" voltage? Where is that from?

I have had issues with these PSUs - a fault with the AC detect causing it to switch off as soon as it was on. Check pin AC-DET it should be 5V continuously but in my case it dropped out a little as the set tried to power on causing it to power off immediately (it's the signal telling it that it's OK to turn on.)

[tds-1541]

Re: LG 50PC3D-UD Power Supply Repair

I am powering the unit up on my workbench. No other boards attached. The AC-DETECT pin measures a constant 5 volts and does not appear to fluctuate.

I pulled the term "Horizontal Grid Voltage" from that LG service manual entitled: lg_50pc1dr_troubleshooting_manual.pdf

VS196v used to drive the Horizontal Grid Voltage, Y SUS, and Z SUS. Measured from pins 8, 9, and 10 of connector P805 to chassis ground. Adjusted at VR951

I am able to enable each section of the power supply except for the high voltage section. As soon as I enable the high voltage section, the VS jumps up over 215v and the power supply kicks into protect mode. I just have to figure out why the VS regulation circuit is not working and what the SD20N60 power MOSFET could have damaged when it died.

Can I use two 125v 50watt incandescent lamps in series to produce a load for VS?

Thanks in advance.

[tom66]

Re: LG 50PC3D-UD Power Supply Repair

Try adjusting the Vs adjustment potentiometer?

Yes 120V lamps or 240V lamps can and have been used as Vs loads. As a note, a glowing black screen uses about 50 watts on Vsus. So you can size the lamps for that low load.

[freakaftr8]

Re: LG 50PC3D-UD Power Supply Repair

Look for a transistor that runs off the vs poteometer think it's bad. Had this happen to a 50pc5d once.

[tds-1541]

Re: LG 50PC3D-UD Power Supply Repair

I have tried adjusting the VS adjustment potentiometer to no avail.

The secondary side of the voltage regulation circuit operates off of a KIA431A voltage reference IC. I was able to verify it's operation in that it does appear to receive 2.47v between the anode and the reference pin. This appears to be where the "adjustment" happens an it is tied to the VR951 potentiometer. From here I am able to verify that the KIA431A then conducts between the cathode and anode which in turn "enables" the optocoupler. I did try and swap both the KIA431A (for an equivalent AS431) and the optocoupler with one taken from an old power supply from my parts bin. This did not change the behavior of the power supply.

I retraced the traces for that optocoupler and it appears to tie into the (vref) pin 5 and the (non-inverting input), pin 8 on the Motorola MC33067P switch mode controller. My initial post stated that it went to pin 10 (fault) which was incorrect. From there, the output runs to pin to pin 3 (Osc Control Curren). This is clearly how the unit determines it's switching frequency and thus the voltage on the secondary side.

I guess I just have to read over that MC33067P datasheet a few more times. I'll also try and diagram the circuit out and post it here. That is unless anyone has any links to the circuit diagrams for this unit? I'll also make a dummy load and test with the load applied.

[Alastair E]

Re: LG 50PC3D-UD Power Supply Repair

I tend to work on PSU's out of the set, and use 100W 240V lamps for the Vs load....

Not had any issues running lamps as a dummy load yest.....

[tds-1541]

Re: LG 50PC3D-UD Power Supply Repair

Well, I was able to get the LG up and running. Turns out the power supply was working just fine after the replacement of that MOSFET. After I replaced that FET, I put the unit back in the chassis and when it failed to power the TV up, I started bench testing it. Turns out you need a load on VS, otherwise the voltage just keeps rising eventual triggering the "protect" circuit. This is not the case for any of the other voltages on this particular power supply apparently. After connecting two 120 volt 50 watt incandescent lamps in series across the VS terminals, the power supply functioned. I let it run for a few minutes. I then shifted my attention to the other components in the chassis.

I had a dead short on the Z-sustain board between VA and ground. Not sure how I missed that in the first place. The Z SUS IPM (Intelligent Power Module) part number YPPD-J015C was shorted internally. I was unable to cleanly remove this component from the Z-sus board (I hate lead free solder). I eventually located an entire replacement board on EBAY for a decent price. Once installed, the TV now powers up and has been displaying the Sochi 2014 Winter Olympics for a good few hours now.

The panel specs 190v for VS, but I set it at 187v. The TV looks great. There are a few very small red sparks/sparkles in the top left and right corners of the display. Very few to be honest, even fewer as the TV warms up. Should I be worried about that? Is that something that I can adjust?

Just to recap: The power supply had a shorted SD20N60 power MOSFET (Q855). I also replaced all 10v 3300uf capacitors with Panasonic FM equivalents. The Z-sus had a bad YPPD-J015C IPM (IC202) so I replaced the board. The manual entitled " lg_50pc1dr_troubleshooting_manual.pdf" was VERY helpful, although it does not contain full schematics.

Hope this helps someone in the future.

[Alastair E]

Re: LG 50PC3D-UD Power Supply Repair

A lot of high current SMPS will do that without a load. The overshoot at turn-on without load then causing the overvolts system to cut in.

The red-sparklies is a known issue and LG issued updated firmware for the control-board to help with this problem.
--Sometimes careful adjustment of SET_Up and a lower Vs can help remove them. Check all voltages for the panel.
(Go too low with the Vs on this chassis, and the PSU cuts out--rather alarmingly with a huge 'bump' from the speakers!)

'Capkid' in the US has the kit to update your control-board if its not possible to adjust them out.

[TBit]

Re: LG 50PC3D-UD Power Supply Repair

Thank you for this very useful post. I replaced all 3300uF 10V capacitors with 16V equivalents, the repair was a success, and I now have one very happy friend watching their 50" plasma.

[pvfjr]

Re: LG 50PC3D-UD Power Supply Repair

Thanks for posting the results of your repair. So many people never follow up, and the thread dies as soon as they find a solution!

I just picked the same TV for free, and it exhibits the exact same problems. I'm curious if it was just the MOSFET that kept your TV from turning on though? I suppose a short on the ZSUS would probably send it into protect mode. Maybe I'll pull my power supply out and bench test it with light bulbs like you did.

Either way, hopefully I'll meet with the same success!

[elviramjw]

Re: LG 50PC3D-UD Power Supply Repair

I have the same issue with a 50pc1da tv, The power supply has a shorted SD20N60 power MOSFET (Q855)
I've ordered some and will post bk later

[elviramjw]

Re: LG 50PC3D-UD Power Supply Repair

changing q855 for a spw20n60c3 worked, tweaked Va & Vs.
However entire screen flickers, cant display menu or change source input.

any suggestions where to look next or to show test screen?

[slidr]

Re: LG 50PC3D-UD Power Supply Repair

Did you ever figure this one out?

[mmartell]

Re: LG 50PC3D-UD Power Supply Repair

changing q855 for a spw20n60c3 worked, tweaked Va & Vs.
However entire screen flickers, cant display menu or change source input.

any suggestions where to look next or to show test screen?

Sounds like main board problem. Try running the test patterns and see how they look.

[hisalive]

Re: LG 50PC3D-UD Power Supply Repair

Good Day, have a similar issue, I've started by pulling and replacing the 10 3300uf caps on the secondary side of the supply, unfortunately I noticed I pulled a 680uf cap that was amongst the 3300's, now I have no idea what spot it goes back into, anyone have a board to check and let me know? Much appreciated.

[ReeceyBurger123]

Re: LG 50PC3D-UD Power Supply Repair

What Diameter was that 680uf cap normally smaller than the 3300's ?

[hisalive]

Re: LG 50PC3D-UD Power Supply Repair

You would think so, but they are practically the same size in diameter and length that's why I never noticed it while pulling them, the 3300's are 10v and the 680 is 50 volts, anyone? Thx